Iron is an essential nutrient that is not only required for carrying oxygen to throughout the bloodstream but is also a vital component of enzymes necessary for oxidative respiration. Iron deficiency is the most common nutrient deficiency in the world, and it is present in 25% of children under the age of 2 (DeMaeyer, E. and Adiels-Tegman, M., World Health Stat. Q. 38: 302-316, 1985; Beard, J. and Stoltzfus, R., J. Nutr. 131: 563S-703S, 2000). The neurological sequelae of chronic, severe, childhood iron deficiency include poor school performance, decreased cognitive abilities, and behavior problems (Beard, J., and Connor, J., Annu. Rev. Nutr. 23: 41-58, 2003; Lozoff, B., et al., Nutr. Rev. 64: S34-S43, 2006), most of which persist following dietary iron supplementation (Felt, B., et al., Behav. Brain Res. 171: 261-270; Lozoff, B., et al., Nutr. Rev. 64: S34-S43, 2006). The brain has the highest rate of oxidative metabolism of any organ and requires relatively high quantities of iron. Dietary iron deficiencies during early postnatal development can result in both mental disorders and severe motor impairments that can persist into adulthood. The process of myelination seems to be particularly dependent on the availability of iron, especially during the critical growth period. (See, Hulet, S. W., et al., J. Neurochem. 72: 868-874, 1999 for citations).
A number of dietary iron supplements have been developed, but have limited efficacy because the iron is poorly absorbed or the supplement is dependent on environmental conditions to generate a consistent level of iron. The standard of care in developed countries is ingestion of 325 mg of ferrous sulfate three times a day. This very high dose of iron is necessary because of the poor absorption of iron in this form. However, because such a high dosing regimen results in gastrointestinal discomfort and a high rate of non-compliance, a need remains for a more efficient and cost-effective oral iron supplement for the treatment of iron-deficiency disorders.
Traditionally, transferrin has been considered the primary mechanism for cellular iron delivery, and a transferrin-mediated transport system has been identified in the blood-brain barrier (Jefferies W. A., et al. Nature 312: 162-163, 1984; Fishman J., et al., J. Neurosci. Res. 18: 299-304, 1987). However, transferrin-independent iron delivery to the brain has been suggested from experiments on hypotransferrinemic mice (Malecki E. A., et al., J. Neurol. Sci. 170: 112-118, 1999), and may involve ferritin.
Ferritin, the main intracellular iron storage protein in many prokaryotes and eukaryotes, is a large (nearly 480 kDa) multi-subunit complex comprising 24 polypeptide subunits. This iron storage complex, found in high concentrations in serum, is capable of containing as many as 4,500 atoms of iron ions (Fe3+) within a hydrous ferric oxide core. In mammals, there are two distinct subunit classes, heavy (H) and light (L) type with a molecular weight of about 21 kDa and 19 kDa, respectively, which share about 54% sequence identity. The H and L subunits appear to have different functions: the L subunit enhances the stability of the iron core while the H subunit has a ferroxidase activity that appears to be necessary for the rapid uptake of ferrous iron. H-rich ferritins are localized in tissues undergoing rapid changes in local ion concentration. For example, expression of the H subunit is preferentially increased relative to the L subunit in cells undergoing differentiation, development, proliferation and metabolic stress.
The brain imposes heightened challenges to iron acquisition because of the highly developed tight junctions that bind neighboring endothelial cells that make up the brain microvasculature. These junctions prevent the paracellular flux of molecules into the brain. The resulting blood-brain barrier is a highly effective mechanism for protecting the brain from potentially harmful substances that circulate in the blood. A consequence of such a blockade, however, is that specific transport mechanisms must be designed for the many trophic substances, such as iron, that are required for normal brain function. In addition, traditional methods of measuring hemoglobin and hematocrit levels in blood samples do not address whether iron is crossing the blood-brain barrier or provide any indication of brain iron concentrations (Beard, et al., J. Neurosci. Res. 79: 254-261, 2005; Malecki, et al., J. Neurosci. Res. 56: 113-122, 1999).
Although H-ferritin has been shown to supply iron to iron-deficient rats, restoration of hemoglobin and hematocrit levels in animals fed H-ferritin in these studies was no better than in animals fed FeSO4, the current standard of care. (Chang, Y-J., et al., Nutrition 21: 520-524, 2005). Therefore, a need remains for an improved method for treating iron-deficiency disorders.